Akio kumada

ABSTRACT

1. A STORAGE DEVICE COMPRISING: A MEMORY ELEMENT INCLUDING A STABLE IRREGULAR FERROELCTRIC BODY COMPOSED OF A CRYSTAL PLATE SELECTED FROM A GROUP OFF GD2(MOO4)3 SINGLE CRYSTAL AND ITS CRYSTALLOGRAPHIC ISMORPHS AND BORACITE WHICH STRAINS AT THE TIME OF POLARIZATION REVERSAL AND IS PROVDIDED WITH A PAIR OF ELECTRODES ON UPPER AND LOWER SURFACES THREOF; A LOAD CONNECTED IN SERIES WITH SAID MEMORY ELEMENT FOR DETECTING THE CHANGE IN POLARIZATION OF SAID MEMORY ELEMENT; DRIVING MEANS FOR APPLYING A WRITE-IN VOLTAGE PULSE AND A READABOUT VOLTAGE PULSE TO THE SERIES CONNECTED CIRCUIT OF SAID IRREGULAR FERROELECTRIC BODY AND SAID LOAD, WHEREIN SAID RESPECTIVE WRITE-IN VOLTAGE PULSE AND SAID READOUT VOLTAGE PULSE EACH HAS A PULSE HEIGHT SUFFICIENT TO CAUSE THE POLARIZATION REVERSAL OF SAID IRREGULAR FERROELECTRIC BODY.

Oct. 1, 1974 AKIO KUMADA Re. 28,179

FERROELECTRIC STORAGE DEVICE USING GADOLINIUH MOLYBDA'I'E Original Filedlarch 25, 1969 4 Shuts-Shoat 1 INVENTOR FA A9 Mum/WM BY M t ATTORNEY JOct. 1, 1974 K o KUMADA Re. 28,179

FERROELECTRIC STORAGE DEVICE USING GADOLINIUM MOLYBDATE Original FiledMarch 25, 1969 4 Sheets-Sheet 2 20L F/G. 4

E k *5 3 5 /0- Lg i E i K.) 1 f U i I r l l 4 L E5()El+) /0 20 AP L/EDF/ELD (KP/cm) R540 /5 PULSE L040 I NVENT OR BY i/M'YJ/z ATTORNEY AKIOKUMADA Oct. 1, 1974 FERROELECTRIC STORAGE DEVICE USING GADOLINIUMMOLYBDATE Original Filed March 25, 1969 4 Sheets-Sheet 4 FIG. /0b

INVENTOR y 4, WW

BY a; (121 ATTORNEY United States Patent 28,179 FERROELECTRIC STORAGEDEVICE USING GADOLINIUM MOLYBDATE Akio Kumada, Kodaira, Japan, assignorto Hitachi, Ltd., Tokyo, Japan Original No. 3,623,031, dated Nov. 23,1971, Ser. No. 810,202, Mar. 25, 1969. Application for reissue Jan. 25,1972, Ser. No. 220,669

Claims priority, application Japan, Mar. 30, 1968, 43/ 20,817 Int. Cl.Gllc 11/22 US. Cl. 340-1732 11 Claims Matter enclosed in heavy bracketsappears in the original patent but forms no part of this reissuespecification; matter printed in italics indicates the additions made byreissue.

ABSTRACT OF THE DISCLOSURE Electrodes for applying a voltage areprovided on upper and lower surfaces of a stable irregular ferroelectricbody such as Gd (MO single crystal which strains at the time ofpolarization reversal and is stable at room temperature or a similarstable irregular ferroelectric body which has a clear threshold voltagefor polarization reversal, to compose a memory element and a loadelement is connected to one of the electrodes in series with said memoryelement. Read-out or write-in pulses having a voltage suflicient tocause the polarization reversal of said stable irregular ferroelectricbody and with a polarity opposite to each other are applied to saidmemory element and load element from a driving circuit, and the changein the polarization of said memory element when the read-out pulse isapplied is read out as a change in voltage across the load element.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an electric storage device employing a ferroelectriccapacitor.

2. Description of the Prior Art A ferroelectric memory is well known inwhich a binary digital signal is memorized by the direction ofspontaneous polarization of a ferroelectric.

Up to now, as ferroelectrics for a memory such materials as bariumtitanate, triglycine sulfate and the like have been considered. Now, aspecial feature of the ferroelectric memory is that a winding woundaround a memory element is not needed as in the case of a ferrite coreusing magnetism, and it is sufficient to provide reticulate electrodeson the upper and lower surfaces of a ferroelectric crystal, so that theconstruction becomes very simple. Therefore, it is known that a memoryelement can be made small in size and is suited for constructing a largecapacity memory. However, memory elements must be arranged in matrixform and the element must be driven by coincident-voltage in order toprovide a large capacity memory; then the disturbance voltage generatedat the time of write-in or readout becomes inevitably one half of thewrite-in or readout voltage. Generally, a ferroelectric memory has sucha character that its memory state becomes unstable after the repeatedapplication of said disturbance voltage since its nonlinearity ofswitching time characteristics is slow. Therefore, when the Writein iscarried out to all elements of a memory matrix having many memoryelements, a defect arises such that the whole memory content becomesunstable. Much effort Reissued Oct. 1, 1974 ice has been made to lowerthe disturbance voltage and has been possible to costruct a matrix planehaving memory elements of about 10 However, when the write-in is carriedout to all of the elements, the same memory content must be writtenagain before the memory state becomes unstable and the record of thememory content disappears; such a memory is not suitable for practicaluse. The cause of the instability of the memory state is considered tobe as follows, namely the value of the coercive field of theferroelectric is strongly dependent upon the frequency and voltage ingeneral and the polarization reversal is caused even by a small backvoltage when it is applied for a long time, that is, the instability isconsidered to be caused by such a property of the ferroclectric that thecoercive field is zero against the gradual change of the electric field.Therefore, there has been for many years a demand for a ferroelectricwhich has a threshold value in the coercive field and does not changehowever often a voltage may be applied, if the voltage is lower than thethreshold value, and much effort has been made by many researchers tofind such a ferroelectric. However, such a ferroelectric has not yetbeen proposed.

Now, the present inventors have found that the strain of a unit cell ofgadolinium molybdate Gd (M0O or its crystallographic isomorphs differsdepending upon the directions of the spontaneous polarization and havepro posed that such a ferroelectric named an irregular ferroelectric byus can be used as an electromechanical transducer. (US. Pat. applicationSer. No. 749,509 filed on Aug. 1, 1968).

It was found by continued study that, in addition to said gadoliniummolybdate (hereinafter referred to as GMO in this specification) and itscrystallographic isomorphs, that is, R R O 3Mo FeO (where R and R are atleast one rare earth element, respectively, and x and e take a value0l.0 and 0-0.2, respectively), such materials as boracite, that is, Me BOX (where Me is a diatomic metal and X is a halogen), KDP (potassiumdihydrogen phosphate), Rochelle salt and the like have also theirregular ferroelectric characteristic.

However, Rochelle salt has such defects namely its Curie point is so lowas 23 C., it is water soluble, weak in mechanical strength and apt to beimpaired by moisture or desiccation. The Curie point of KDP also is sucha low temperature as -1SO C. Therefore, the presently known irregularferroelectrics which are stable at room temperature are GMO and itscrystallographic isomorphs and boracite.

These stable irregular ferroelectrics have a clear threshold value ofapplied voltage at which is caused the polarization reversal and arebest suited for the ferroelectric memory.

SUMMARY OF THE INVENTION The main object of the present invention is toprovide a ferroelectric memory which is stable in operation.

Another object of the present invention is to provide a small-sized butlarge capacity memory.

A further object of the present invention is to make it possible toincorporate an irregular ferroelectric into a memory.

Therefore, the present invention comprises essentially a storage devicecomprising a memory element composed of an irregular ferroelectric bodywhich strains at the time of polarization reversal and is stable at roomtemperature and a pair of electrodes provided on the upper and lowersurfaces of said irregular ferroelectric body for applying a voltage tosaid irregular ferroelectric body, a load connected in series with thememory element for detecting the change of polarization of said memoryelement and a driving means for applying a write-in or readout voltage,each having an opposite polarity, between said pair of electrodes ofsaid memory element.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are diagrams forillustrating the strain of an irregular ferroelectric at the time of thepolarization reversal, respectively;

FIGS. 3a, 3b and 4 are diagrams illustrating that the irregularferroelectric has characteristics different from an ordinaryferroelectrics, respectively;

FIGS. 5a, 5b, 6a, 6b and 7 are diagrams illustrating the construction ofcircuits of different embodiments according to the present invention,respectively; and

FIGS. 8, 9, 10a, 10b and 11 are diagrams illustrating the constructionsformed in matrix form of different embodiments of the present invention,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagramillustrating that an irregular ferroelectric strains at the time ofpolarization reversal. In the figure, rectangles shown by a solid lineor dotted line indicate a single crystal plate of an irregularfcrroelectric, and showing a plane view of a surface of the crystal cutalong the (001) plane, so-callcd C-plane and further cut along the(ll0)clcavage plane seen from the direction parallel with the c-axis.Here, the GMO crystal is explained as an example of an irregularferroelectric. The

unit cell of GMO single crystal is orthorhombic with point symmetrymntZ, and its a, b and c-axis were measured by X-ray diffraction methodusing an X-ray goniometer and the result is as follows:

a:10.388i0.005 A., b: 10.426i0.005 A., c:l0.709i0.005 A.

Upper and lower surfaces of zcut surface of the GMO crystal are groundand electrodes are provided on the whole surfaces. Thus, an electricfield can be applied to the GMO crystal in the direction of the c-axis.

Now, when negative and positive voltages are applied on the outside andinside of the surface of paper on which GMO crystal plate is drawn,respectively, the GMO crystal plate is reversed spontaneously inpolarization in the direction from the inside to the outside of thepaper and comes to the state 1 shown by the solid line in the figure.Next, contrary to the above, when the positive voltage is applied on theoutside of the paper and the negative voltage is applied on the insideof the paper, the GMO crystal is reversed spontaneously in polarizationin the direction from the outside to the inside of the paper and isdeformed into the state 2 shown with the dotted line in the figure. Thisdeformation interchanges the a-axis and b-axis and in the figure achanges into b and b changes into a. When the strain caused by saiddeformation is defined by X,:ab. a+b, and X is about 1.5 X10" in thecase of the GMO crystal and the strain can be measured directly by themethod described above.

FIG. 2 is a diagram showing one mode of the deformation of an irregularferroelectrics plate. The deformation is shown with exaggeration inFIGS. 1 and 2.

The crystal plate of FIG. 2 is also a 45 z-cut plate as the crystalplate shown in FIG. 1 (a plate in parallel with each surface indicatedby Miller indices (001), (110) and (110). As can be seen from FIG. 1,the GMO crystal and the like do not change their length in the surface(110) and (110) at the time of the polarization reversal. Therefore,when the polarization reversal is caused by a vo'itage applied to theelectrodes provided on the upper and lower surfaces of the z-cutsurface, the shear strain is caused as shown in FIG. 2. The deformationcaused by the shear strain has generally such a deformation mode asshown in the figure, where a nucleus is produced at the periphery of thecrystal and the deformation gradually reaches the center of the crystal,thus the crystal deforms as a whole. When said nucleus of thedeformation is too small, the deformation cannot be maintained due tothe force from the neighboring crystal, thus said nucleus of thedeformation must have a certain magnitude. For cxamplc, the width of thenucleus 3 produced in the crystal shown in FIG. 2 must be larger than acertain value. In order to cause such a deformation largcr than acertain magnitude, an energy more than a certain value may be required,and it can be considered that this is the reason why an irregularferroelcctric which strains at the time of. the polarization reversalhas a threshold value in the voltage applied for causing thepolarization inversion.

FIGS. 3a and 3b are diagrams of measuring circuit and output waveformfor illustrating the characteristics of an irregular ferroelectric, andFIG. 4 is its characteristic diagram.

Referring to FIG. 3a, reference numeral 4 is an alternating voltagesource, 5 a slidac, 6 a variable resistance, 7 a crystal to be measured,which is, for example, GMO single crystal, 8 an output condenser and 9an oscilloscope for observing the fcrroelectric hysteresis loop(hereinafter referred to as D-E loop). Said GMO single crystal 7 is az-cut plate having a thickness of 0.2 mm. and a size of 1 mm. X 1 mm.,and its upper and lower surfaces are ground, then transparent electrodesare provided on the whole surfaces. This single crystal 7 is placedunder a polarizcd microscope (not shown) in order that change in domainsis observed with crossed polarization.

When a 50 Hz. AC voltage is applied to the GMO single crystal 7 from thealternating voltage source 4, the D-E loop of the GMO single crystal 7can be observed. Now, when the voltage applied to the single crystal 7is increased by rotating the slidac, domains of the crystal 7 do notchange if the peak-to-peak value of the applied voltage is lower than280 v. and only a line 10 appears on the surface of the oscilloscope 9as shown in FIG. 3b. Then, when the applied voltage is made about 280 Vpeak-to-peak value, said domains change suddenly and a DB loop indicatedby 11 appears on the surface of the oscilloscope 9, and when the appliedvoltage is increased further, a hysteresis loop as indicated by 12 canbe observed. When the voltage is decreased, this hysteresis loop isobserved up to an applied voltage of 200 v.,, and when the voltage ismade lower than the value, the hysteresis loop disappears suddenly. Thisphenomenon indicates that GMO single crystal 7 has a threshold value E,in the applied voltage necessary for causing the polarization reversalat room temperature. This threshold electric field E can be determinedby the relation between the coercive field and applied field of the D-Eloop shown in FIG. 4 which is drawn based on the observed result shownin FIG. 3b. That is, it can be seen from the figure that the thresholdelectric field (E (-l-) at rise time is about 7 kv./cm. and thethreshold electric field E at fall time is about 5 kv./'cm. Therefore,when the GMO crystal is used as ferroelectrics memory, a stable storageoperation can be carried out by applying an electric field of more than7 kv./cm. to the crystal at the time of readout and writein and makingthe disturbance voltage lower than 5 kv./ cm. Here, said thresholdelectric field is a value in the case that electrodes are provided onthe upper and lower surfaces in their entirety of said single crystal.However,

almost the same results were obtained with many other GMO samples whenidentical electrodes were provided, according to the experiments by thepresent inventor. Further, it was shown by said experiments that whenelectrodes are provided at a portion of the upper and lower surfaces ofthe single crystal, the threshold electric field differs depending uponthe shape of the electrodes and the angle to the crystallographic axis.Furthermore, transparent electrodes were used in the above experimentsfor the purpose of observation, of course opaque electrodes can also beused.

FIGS. 5a and 5b are diagrams illustrating the construction of anembodiment of the present invention, wherein FIG. 5a shows an examplewhere said irregular ferroelectric crystal, for example, the GMO crystalis used as a memory element and the operation of write-in and readout iscarried out by a voltage pulse, and FIG. 5b is a diagram showing apolarization P versus applied voltag V hysteresis loop of said GMOcrystal. Referring to FIG. 5a, reference numeral 13 is a memory elementcomposed by providing electrodes on the 45 z-cut surfaces of a GMOsingle crystal of 0.2 mm. in thickness, 14 a load connected in serieswith said memory element 13, 15 a terminal for applying the write-in andreadout voltage and 16 an output signal terminal.

In order to write a digital signal 1 into the memory element 13comprising the GMO single crystal, a positive voltage pulse is appliedto the memory element 13 through the terminal 15. That is, when apositive voltage pulse of 150 v. is applied to the memory element 13 inthe state of P, shown in FIG. 5b (the state stored as through theterminal 15, the polarization reversal is cause in the GMO crystal andthe state of spontaneous polarization +P shown in FIG. b is produced;thus 1 is stored. Now, when said positive voltage pulse is furtherapplied to the state 1, the polarity of the spontaneous polarization +Pdoes not change and the stored content 1" does not change. In order towrite a digital signal 0, a negative pulse of 150 v. is applied to saidmemory element.

The readout of stored contents can be carried out by applying a negativevoltage pulse of 150 v. to the terminal through the readout load 14 inthe circuit shown in FIG. 5a. That is, when l is stored, thepolarization reversal is caused in the GMC) crystal by the appliednegative voltage pulse and the change in impedance of the memory element13 caused by the polarization reversal appears at the output terminals16 as the change in voltage across the load 14, and when 0 is stored,the polarization reversal is not caused by said applied negative voltagepulse and the impedance change of the memory element does not result;then only a small output is observed. Thus, stored contents 1 and 0 canbe distinguished.

In the above embodiment, the voltage applied to the memory element atthe time of write-in and readout was 150 v., then the electric field was7.5 kv./cm., the present invention is not limited to this value, thewritein and readout can be carried out by a suitable electric field, ifthe electric field were more than the threshold electric field E of saidmemory element (as described before, this value differs depending uponthe provided electrodes), that is, in this case more than 7 kv./cm. Thewrite-in and readout voltage can be made smaller by making the thicknessof said GMO crystal less than 0.2

FIGS. 6a and 6b show another embodiment of the present invention, inwhich FIG. 6a is a circuit diagram wherein a resistor is used as theload in FIG. 5a, and FIG. 6b is a diagram showing a readout outputwaveform of the circut shown in FIG. 6a. Referring to FIG 6a, if thesignal 1" is stored in the memory element 13 at the time when thenegative voltage is applied to the terminal 15, an output signalindicated by l in FIG. 6b is produced at the output terminal 16 as thechange in voltage across the resistor 14. Onothe other hand, When thesignal 0 is stored in said memory element 13, an output signal indicatedby 0 in FIG. 6b is produced at said output terminal 16. Therefore, if apulse is produced at a time 7' indicated by dotted lines in FIG. 6b tocarry out a sampling. the stored content 1" or 0" can be clearlydistinguished.

In FIG. 621. there is shown a case where the resistor is used as theload, but the load is not limited to resistive load and such loads canbe used as a capacitive load, a resistive-capacitive load or a loadusing a diode or transistor in parallel, and the output signal waveformdiffers depending upon each load.

FIG. 7 is a diagram showing a circuit construction of a furtherembodiment of the present invention. In the figure, two electrodes ofthe memory element 13 are connected to a DC power source 59 throughresistors 53 and 54, respectively, and further connected to collectorsof transistors 55 and 57, respectively. Emitters of said two transistorsare connected together and grounded. Then, when a positive pulse isapplied to the base 58 of the transistor 57, the transistor 57 is madeto conduct and the lower electrode of the memory element 13 is grounded;thus. a voltage is applied to the memory element 13 via the DC powersource 59 and the resistor 53 and the digital signal 1 is written. Onthe other hand, when a positive voltage is applied to the base 56 of thetransistor 55, the transistor 55 is made to conduct and the upperelectrode of the memory element 13 is grounded earthed; thus, in thiscase. a voltage opposite to the one before is applied to said memoryelement via the DC power source 59 and the resistor 54 and the storedcontent of the memory element 13 is read out.

The construction shown in FIG. 7 is advantageous in that a transistorhaving a relatively low breakdown voltage can be used as saidtransistors 55 and 57 since a voltage much higher than the thresholdvalue of the memory element 13 is not applied to the transistors 55 and57.

FIG. 8 is a diagram showing a circuit construction of still anotherembodiment of the present invention, in which the driving method differsfrom the embodiments of FIGS. 5 and 6. In the embodiment shown in FIG.8, a terminal 17 is provided in addition to the write-in and readoutterminal 15. Pulse signals having a polarity opposite to each other areapplied to the terminal 15 and terminal 17, respectively. The pulseheight of these two pulses is so determined that each pulse alone isinsufficient to reverse the polarization of the memory element 13 (thatis, in the case of said GMO crystal, it is lower than 5 kv./cm.) andwhen two pulses are applied at the same time, the superimposed pulse issufficient to reverse the polarity of the memory element 13 (that is, inthe case of the GMO crystal, it is higher than 7 kv./cm.). Usually, twovoltage signals having the same height and opposite polarity are chosenas said two pulse signals, for simplicity.

For example, when the memory element 13 is made of a z-cut GMO singlecrystal of 100, in thickness, a pulse of +45 v. is applied to theterminal 15 and another pulse of 45 v. is applied to the terminal 17(the electric field applied to the memory element 13 is 4.5 kv./cm. wheneach pulse alone is applied, and it becomes 9 Irv/cm. when the twopulses are applied at the same time) at the time of the write-in of thedigital signal I," and a pulse of 45 v. is applied to the terminal 15and another pulse of +45 v. is applied to the terminal 17 at the time ofthe readout, thus the operation of the writein and readout can becarried out.

Therefore, the output signal produced at the output terminal 16 differsdepending upon the stored content of the memory element 13 at the timeof the readout, as described before.

By the way, a diode 18 provided between the connection point of thememory element 13 and load 14 and the output terminal 16 is a means toprevent the negative voltage applied to the terminal 17 at the time ofthe write-in appearing directly at the output terminal 16, and theomission of it does not affect the operation of the memory.

As described above, the memory element 13 can be drive selectivity inthe embodiment of FlG. 8, since the readout or write-in operation iscarried out only when two driving pulses of opposite polarity areapplied simultaneously to the terminals and 17, respectively.

FIG. 9 is a diagram illustrating another embodiment of the presentinvention, in which a plurality of unit memory circuits shown in FIG. 8are disposed in matrix form. In the figure, the portion 19 encircled bya dotted line is a matrix of memory elements and its size 3 x 3.

The matrix can be made in a desired suitable size, and

in general, a matrix of i x j can be constructed. Reference numerals20-28 indicate memory elements constructing the matrix 19, each of whichis composed by providing electrodes on both surfaces of the z-cutsurfaces of said irregular ferroelectrics, for example, a GMO singlecrystal.

All of the memory elements 20-28 are connected in such a manner thatupper electrodes of the elements in each row of the matrix are connectedtogether and connected further to X-drive lines 29-31, respectively, andlower electrodes of the elements in each column of the matrix areconnected together and connected further to Y-clrive lines 32-34,respectively. That is. upper electrodes of the memory elements 20-22,23-25, and 26-28 are connected commonly to the X-dt'ive lines 29, and31,

respectively, and lower electrodes of the memory elements (20, 23, 26),(21, 24, 27), and (22, 25, 28) are connected commonly to the Y-drivelines 32, 33 and 34, respectively.

Said Y-drive lines 32-34 are connected to one end of resistors -37provided as loads, respectively, and another end of said resistors 35-37is connected to Y-drive circuits -52, respectively and a driving pulsebeing applied selectively to each Y-drive line. The X-drive lines 29-31are connected to Xdrive circuits 38-40. respectively, and the drivingpulse is selectively applied to each X- drive line. The Y-drive lines32-34 are connected to resistors 35-37, respectively, and furtherconnected to a sense circuit 42. Said Y-drive circuits 50-52 areprovided to each Y-drive line and apply the driving pulse selectively toeach Y-drive line. However, in the case of. parallel readout, onedriving pulse can be applied to each Y-drive line from one drivingcircuit.

Two driving pulses of opposite polarity are selectively applied to theX-drive line and Y-drive line, respectively, by means of theconstruction described above, and a memory element provided at thecrossed position is driven. For example, when the positive pulse isapplied to the X-drive line 29 and the negative pulse is applied to theY-drive line 33 at the same time, the digital signal 1" is written intothe memory element 21. Further, when the negative pulse is applied tothe X-drive line 30 and the positive pulse is applied to the Y-driveline 32 at the same time, the voltage produced across the resistor 35differs depending upon the stored content 1 0r 0 of the memory element23, and the difference is detected by the sense circuit 42. The write-inand readout to other memory elements can be carried out by suitablyselecting the X-drive line and Y-drive line.

As can be seen from the above description, a stored content of thememory element according to the present invention is destroyed when itis read out. Therefore, it is required to carry out a rewrite-in inorder to keep the ill till

stored content of the memory element after the readout. For example,when a stored content of the memory element 28 is read out by applyingthe negative pulse to the X-drive line 31 and the positive pulse to theY-drive line 34 simultaneously and it is detected by the sense circuit42, the stored content of the memory element 28 is destroyed.Accordingly, if the rewrite-in is carried out by applying the write-indriving pulse to the X-drive line 31 and Y-drive line 34, depending uponthe stored content of the memory element 28 detected by the sensecircuit 42, the stored content can be retained. At this time, theoperation to rewrite the digital signal 1 into said memory element 28 isthe same as the write-in of 1" described before. The operation ofrewrite-in of the digital signal 0" to said memory element 28 is carriedout by applying the negative pulse to the Y-drive line 34 and also tothe X-drive line 31, or not applying the pulse at all. This is becausesaid memory element 28 is already in the state of 0" at the time of thereadout, then it is only necessary to keep said memory element 28 in thesame state without causing the polarization reversal.

Thus, when the rewrite-in operation is carried out as described above,the negative driving pulse is applied to the Y-drive line in each caseof the rewrite-in of the digital signals 1 and 0," and one of thepositive driving pulse and negative driving pulse (or no driving pulse)can be selected on the side of the X-drive circuit depending upon thestored content 1" or 0," and, therefore, the operation of the drivingcircuits 38-40 and 50-52 can be simplified.

In the case of the write-in operation, the same driving method as saidrewrite-in can be used if such method is utilized that the readout isalways carried out before the write-in.

As to the driving circuits 38-40, 50-52 and the sense circuit 42 shownin FIG. 9, a known circuit as the driving circuit and sense circuit of aferroelectric memory or magnetic core matrix can be used and theirdetails are omitted here.

FIGS. 10a and l0b are diagrams illustrating an example of theconstruction of a memory matrix used in the storage device of thepresent invention, wherein FIG. 10a is a perspective view, and FIG. 10bis a sectional view along the line AA in FIG. 10a. Said memory matrixcorresponds to the memory matrix portion 19 in FIG. 9, and

other portions can be constructed by utilizing the same circuit.

In the figures, reference numeral 43 indicates a 45 z cut plate of anirregular ferroelectric crystal, for example, a GMO single crystal. Inthe GMO single crystal, the plane indicated by Millers indices (001) and(110) is the cleavage plane, so that the crystal is apt to cleave alongsaid plane. Therefore, when a z-ctlt plate of GMO single crystal groundto a thickness of g is cut by an ultrasonic cutter or diamond cutterinto a width of 100a parallel to said plane and a plane perpendicular tosaid plane, a crystal plate 43 having meshshaped crack 44 is obtained asshown in the figures. On the surface of the crystal plate 43, electrodes45 having a width of about lOOu are provided with a space of about 100Thus, said electrode 45 is provided to cover a line of mesh entirely onevery other mesh of said crystal plate 43. Electrodes 46 are provided onthe back surface of said crystal plate 43 in the direction perpendicularto said electrode 45 in a shape which is the same as with the facesurface. A small mesh of crystal positioned at an intersecting point ofsaid electrodes 45 and 46 forms a memory element by such construction.

Said electrodes 45 and 46 can be provided by evaporating a metal such asaluminum through a mask placed on the crystal plate 43. A transparentelectrode including indium dioxide InO as a principal ingredient can beformed by evaporating indium metal in vacuum on the crystal plate 43 andperforming heating oxidation in the atmosphere or anodic oxidation byelectrolysis. A transparent electrode can also be formed by spraying aliquid of tin tetrachloride SnCl instead of said metal on the crystalplate 43 maintained at a temperature of about 500 C. for creating byreaction a transparent electrode including tin dioxidle SnO as principalingredient, and cooling it gradually.

A 100 x 100 memory matrix can be formed on a 2 cm. x 2 cm. GMO crystalplate by the method described above.

In FIGS. a and 10b there were shown an example in which is used a GMOsingle crystal as an irregular ferroelectric. As described above, theGMO single crystal is accompanied by a strain of about X :1.5 at thetime of the polarization reversal, so that a partial deformation cannotbe caused in a crystal plate. Therefore, it was necessary to cut thecrystal in mesh shape as shown in FIGS. 10a and 10b. However, saidstrain X of an irregular ferroelectric such as boracite is one ordersmaller than that of GMO single crystal, so that partial deformation canbe caused in a crystal. When such a crystal is used, the mesh-shaped cutas shown in FIGS. 10a and 10b is not necessary, and the construction ofa memory matrix can be considerably simplified.

FIG. 11 is a diagram illustrating another example of the construction ofthe memory matrix according to the present invention, which differs fromthe embodiment of FIGS. 10a and 10b in that a memory matrix is providedon an insulator substrate 47. The structure of the memory matrix shownin FIGS. 10a and 10b is very weak since the thin GMO crystal is cut inmesh shape. Therefore, it is effective to bind it on the insulatorsubstrate having a sufficient thickness. Also, in the case ofmanufacturing said memory matrix, such a method is effective as firstthe electrode 46 is formed on the back surface of the GMO crystal plate,next the plate is bound on said insula tor substrate 47, then said saidGMO crystal plate is cut in mesh-shape from the face surface thereof andthe face electrode 45 is provided. Though the present invention has beendescribed above in conjunction with an embodiment using the GMO singlecrystal as an example, the present invention is not limited to the GMOsingle crystal, and boracite and other stable irregular ferroelectricscan be used by the same construction.

What is claimed is:

1. A storage device comprising: a memory element including a stableirregular ferroelectric body composed of a crystal plate selected from agroup of Gd (MoO single crystal and its crystallographic isomorphs andboracite which strains at the time of polarization reversal and isprovided with a pair of electrodes on upper and lower surfaces thereof;

a load connected in series with said memory element for detecting thechange in polarization of said memory element;

driving means for applying a write-in voltage pulse and a readoutvoltage pulse to the series connected circuit of said irregularferroelectric body and said load, wherein said respective write-involtage pulse and said readout voltage pulse each has a pulse heightsufficient to cause the polarization reversal of said irregularferroelectric body.

2. A storage device according to claim 1, wherein said irregularferroelectric body consists of Gd (MoO 3. A storage device according toclaim 1, wherein said irregular ferroelectric body consists ofcrystallographic iSOmOl'ph Of Gd22(M0O4)3- 4. A storage device accordingto claim 1, wherein said irregular ferroelectric body consists ofboracite.

5. A storage device comprising: a plurality of memory elements arrangedin rows and columns into a matrix shape wherein each memory elementcomprises a stable irregular ferroelectric body made of a GMO singlecrystal plate which strains at the time of polarization reversal and isprovided with a plurality of electrodes on face and back surfacesthereof cut crosswise in mesh shape in two directions intersecting eachother, said plurality of electrodes covering a mesh of said GMO singlecrystal plate at the place of each crossing point thereof, and whereinone of said pair of electrodes of said memory elements in each row ofsaid matrix is connected commonly to form X-drive lines, respectively,and the other one of the pair of electrodes of said memory elements ineach column of the matrix is connected commonly to form Y-drive lines,respectively; loads connected in series to Y-drive lines, respectively,for detecting the change in polarization of said memory element; anddriving means for selectively applying two driving pulses of oppositepolarity to one of the X-drive lines and to one of the Y-drive linesthrough said loads, respectively, the pulse height of each of said twodriving pulses being insufficient to reverse the polarization of saidirregular ferroelectric body, while a superimposed pulse height of saidtwo driving pulses is sufficient to reverse the polarization of saidirregular ferroelectric body.

6. A reversibly polarized storage device comprising:

a crystal of a material selected from the group consisting Gd (MoO andits crystallographic isomorphs and boracite;

electrodes connected to said crystal; and

means for applying a write-in voltage pulse and a readout voltage pulseto said electrodes for selecting one of the reversible polarizationstates of the crystal.

7. A bistable storage element comprising:

a reversibly polarizable crystal of a material selected from the groupconsisting of Gd (MoO and its crystallographic isomorphs and boracitehaving a ferroelectric axis,

electrodes connected to said crystal;

means for applying a write-in voltage pulse and a readout voltage pulseto said electrodes, the polarity of said pulses corresponding to thedirection of polarization; and

means for detecting the bistable stable of said crystal by sensing thesign of said polarization.

8. A ferroelectric switching element comprising:

a reversibly polarizable crystal of boracite having a ferroelectricaxis, said crystal having two opposing parallel faces that are rightangles to said ferroelectric axis:

electrodes disposed on said parallel faces; and

means for applying voltage pulses to said electrodes,

the polarity of said applied voltage pulses determining the direction ofpolarzaton after ther termnation.

9. A reversibly polarizable storage device comprising:

a crystal of a material selected from the group consisting of gadoliniummolybdate, the crystallographic isomorphs thereof, and boracite; and

means for selectively writing information in and reading information outsaid crystal comprising:

first and second electrodes disposed on opposite planar surfaces of saidcrystal, and

means for selectively applying a write-in voltage pulse and a read-outvoltage pulse, the magnitude of which is at least equal to the voltagewhich causes a reversal of the polarization state of said crystal, tosaid electrodes, so as to selectively place said crystal in one of thereversible polarization states thereof and to subsequently read out thestate of polarization of said crystal exclusively by the application ofa selected voltage pulse to said electrodes.

10. A reversibly polarizable storage device according to Claim 9,wherein said crystal is made of boracite.

11. A reversibly polarizable storage device according to Claim 9,further including means, connected in series with one of said electrodesof said crystal, for detecting References Cited The followingreferences, cited by the Examiner, are of record in the patented file ofthis patent or the original patent.

UNITED STATES PATENTS 10 Matthias 340-1732 X Anderson 340-1732 Fries340-173.). X

Pulvari 340-1731 X Pulvari 340-173.2 X 15 OTHER REFERENCES Borchardt eta1. Ferroelectrie Rare-Earth Molybdates, April 1967, Journal of AppliedPhysics, Vol. 38, No. 5, pp. 2057-2060.

Borchardt et a1, Gd (MoO A Ferroelectric Laser Host, Jan. 15, 1966,Applied Physics Letters, Vol. 8, No. 2, pp. 50/52.

STUART N. BECKER, Primary Examiner

1. A STORAGE DEVICE COMPRISING: A MEMORY ELEMENT INCLUDING A STABLE IRREGULAR FERROELCTRIC BODY COMPOSED OF A CRYSTAL PLATE SELECTED FROM A GROUP OFF GD2(MOO4)3 SINGLE CRYSTAL AND ITS CRYSTALLOGRAPHIC ISMORPHS AND BORACITE WHICH STRAINS AT THE TIME OF POLARIZATION REVERSAL AND IS PROVDIDED WITH A PAIR OF ELECTRODES ON UPPER AND LOWER SURFACES THREOF; A LOAD CONNECTED IN SERIES WITH SAID MEMORY ELEMENT FOR DETECTING THE CHANGE IN POLARIZATION OF SAID MEMORY ELEMENT; DRIVING MEANS FOR APPLYING A WRITE-IN VOLTAGE PULSE AND A READABOUT VOLTAGE PULSE TO THE SERIES CONNECTED CIRCUIT OF SAID IRREGULAR FERROELECTRIC BODY AND SAID LOAD, WHEREIN SAID RESPECTIVE WRITE-IN VOLTAGE PULSE AND SAID READOUT VOLTAGE PULSE EACH HAS A PULSE HEIGHT SUFFICIENT TO CAUSE THE POLARIZATION REVERSAL OF SAID IRREGULAR FERROELECTRIC BODY. 